The present invention is generally directed to optical communications, and more specifically to improved methods of increasing the information transmission capacity for a single fiber.
Historically, several steps have been taken to improve the information transmission bandwidth in single mode fiber (SMF) optical communications systems, which are typically used for transmitting information over distances of a kilometer or more. Low transmission loss silica fibers were developed in the late 1970s and early 1980s, permitting the use of silica fibers over greater distances. The advent of erbium-doped fiber amplifiers (EDFAs), providing amplification for signals around 1550 nm, permitted the transmission of signals over even greater distances, while the introduction of wavelength division multiplexing/demultiplexing (WDM) extended the bandwidth of silica fibers by permitting a single mode silica fiber to carry different optical signals at different wavelengths. Optical communication systems have further benefitted from the introduction of advanced techniques such as polarization multiplexing and higher order modulation schemes to increase spectral efficiency (bits/s/Hz). However, current SMF optical transmission systems are now approaching their intrinsic capacity limits, and it is expected that they will be unable to meet future capacity requirements.
One approach being considered for increasing fiber capacity is space division multiplexing (SDM), in which different optical signals are physically (spatially) separated from each other within the same fiber. One particular implementation of SDM is to use a multi-core fiber (MCF), in which a number of different single-mode cores are contained within the same cladding material, laterally separated from each other within the cladding. An important issue for MCF is that crosstalk between cores or modes increases with transmission distance, and/or arises due to bends and fiber imperfections. Extensive digital signal processing is, therefore, needed to perform channel characterization and cope with the crosstalk in a fashion similar to multiple-input multiple-output (MIMO) transmission in radio systems. Furthermore, it is difficult and expensive to manufacture optical fibers having multiple cores within a single cladding. Furthermore, connectivity of the MCF is complicated because the multiple cores require precise rotational alignment of the fiber end about the fiber axis in order for the cores to be aligned.
Another proposed implementation of SDM relies on a fiber having a single core with a diameter that is larger than required for single-mode operation and which supports the propagation of a small number of modes. This fiber is referred to as a few-mode fiber (FMF). In a perfectly straight and circularly symmetric fiber, the modal electromagnetic fields do not interact in the sense that the power carried by each mode remains unchanged as the total electromagnetic field propagates in the fiber, thus theoretically each mode can act as an independent transmission channel. However, due to fiber imperfections and/or bends, a mode couples power to other modes, predominantly to those that have similar propagation coefficients. Over long distances, the optical power is likely to be distributed over multiple modes. This can be problematic, however, because a mode couples to a specific linear combination of all FMF modes, and the excitation of another mode couples to a linear combination of all FMF modes that is still orthogonal. With the aid of digital signal processing, the original signals can thus still be recovered. The refractive index profile of a typical FMF has a parabolic shape in the core region, to mitigate differential mode delay, i.e., to assure that the arrival times of all the modes are very similar. This relaxes the requirements on the size of the digital signal processor (DSP) required for signal analysis at the receiver.
Another proposed implementation of SDM relies on optical angular momentum (OAM) multiplexing in a fiber. Difficulties with this approach include the implementation of mode (de)multiplexers having high mode selectivity and avoiding the 1/N insertion loss associated with cascaded beam splitters.
Accordingly, there is a need for improved methods of implementing SDM that can reduce the effects of the problems discussed above.